1. Field of the Invention
The present invention relates to a network construction apparatus, method and program for constructing, as a network structure, a specific structure detected from image data.
2. Description of the Related Art
In recent years, high-quality 3D images are used in image-based diagnosis supported by an advance of medical equipment (for example, a multi-detector CT, or the like). Since a 3D image is composed of many 2D images, which have a large data amount, doctors need long time in some cases to find desirable observation regions to diagnose patients. Therefore, some attempt is being made to improve the efficacy of diagnosis by improving the visibility of a whole organ or a lesion by extracting an organ of interest, and by displaying the extracted organ of interest by MIP, VR, CPR, or the like. For example, when a chest CT image is analyzed, and especially when the lung is analyzed, extraction of a center path of a pulmonary artery and a center path of a pulmonary, as independent network structures, is requested.
S. Nakamura et al., “Automated Classification of Pulmonary Artery and Vein from Chest X-ray CT Images by Tree Structure Analysis”, Technical Report of IEICE (the Institute of Electronics Information and Communication Engineers), MI2005-134, [online], [searched on 2010], Internet URL:http://www.murase.nuie.nagoya-u.ac.jp/˜ide/res/paper/J05-kenkyukai-snaka-1.pdf (Non-Patent Document 1) discloses a method for constructing network structures. In the method, blood vessels are extracted, as network structures, from an image of a lung region, and the extracted network structures, which are intermingled with each other, are constructed as independent network structures corresponding to a pulmonary artery and a pulmonary vein. Specifically, segments are generated from each of the structures of blood vessels, and directed network structures of the generated segments are constructed, in directions away from the hilum of lung, from portions of the pulmonary artery and the pulmonary vein that are close to the hilum of lung, as start points. Further, with regard to a problem that pulmonary veins or pulmonary arteries are not correctly connected to each other, Non-Patent Document 1 discloses the following network construction method. The pulmonary veins or pulmonary arteries are not correctly connected to each other because a part of the pulmonary artery and a part of the pulmonary vein that should be extracted as separate segments are extracted as a segment (hereinafter, referred to as a cross segment in some cases) at a position in which the pulmonary artery and the pulmonary vein are in contact with each other. For this problem, Non-Patent Document 1 discloses construction of the directed network structure, in which the direction of the course of a blood vessel is defined at a core line branch of the blood vessel. With respect to a vertex having an indegree greater than or equal to 2, a cross segment is divided at the vertex or at the endpoints of edges stemming from the vertex, and plural segments in the neighborhood of the cross segment are reconnected to each other so that the inner product of a combination of segments is maximized.
However, the method disclosed in Non-Patent Document 1 needs information about the location of a root of the network structure, in addition to the information about each of the extracted segments, to identify the forward direction of the directed network structure. Therefore, when the direction of the network structure is not known, or when the direction of the network structure is erroneously selected, the method disclosed in Non-Patent Document 1 is not able to correctly construct a network structure.